How to drive SiC MOSFET…. The right way !!

Hi. Welcome to Texas Instruments High Volt Interactive session. I am Suxuan Guo, and I will be discussing how to drive silicon carbide MOSFETs the right way. Here is what you will get out of this presentation.
First, why is silicon carbide considered to be the next revolution in power? Second, what are the system benefits enabled by silicon carbide power switches? Third, which applications are the early adopters of the technology? And fourth, what are the critical driver requirements to effectively drive silicon carbide MOSFETs?
During the presentation, I will discuss briefly about TI's silicon carbide driver products, namely UCC2152X family, UCS53XX family, ISO585X family, and UCC2753X family. I will also discuss about some of the relevant end-equipments to the topic, namely solar inverter, HEV/EV traction inverter, EV on-board charger, and off-board charge, also called the charging pile.
Here is a detailed outline of my presentation. I will start by discussing the unique combination of intrinsic material properties of silicon carbide and how they influence power systems. Then I will discuss the silicon carbide application landscape. Next, I will discuss use of silicon carbide in solar and automotive systems. Finally, I will discuss regarding some of the critical driver requirements to effectively drive silicon carbide MOSFETs, and a talk by briefly mentioning TI's silicon carbide driver product families.
Here are a few interesting real life applications of silicon carbide. Silicon carbide is used to manufacture the trauma plates inside ballistic vests, taking advantage of its diamond-like strength. Carbon ceramic, which is essentially silicon carbide, is used to develop car disk brakes. Silicon carbide is also used in steel manufacturing, due to its very high melting point and high thermal conductivity.
A very interesting application of silicon carbide is to make engagement rings. This is attributed to its diamond-like lattice structure of silicon carbide. Being a wide-bandgap material, silicon carbide is used to develop ultraviolet LEDs. Last, but not the least, silicon carbide it has emerged as a superior material to develop high voltage power devices. Interestingly, all these applications, including high voltage power devices, are enabled by a common set of intrinsic material properties of silicon carbide, which we will study in detail next.
Here's a table that compares some of the important intrinsic material properties of silicon, gallium nitride, and silicon carbide from power switch perspective. Silicon carbide, as an intrinsic material, has a higher bandgap and significantly higher breakdown voltage as compared to silicon. Hence, it's a perfect material suited for high voltage applications.
Silicon carbide has a slightly lower electromobility, but almost twice saturation velocity as compared to silicon. Silicon carbide also has almost three times the thermal conductivity as silicon. Let's now understand how these intrinsic material properties translate to power system benefits.
As I mentioned earlier, a higher bandgap energy and breakdown voltage translates into robust high voltage operation. Higher saturation velocity allows for faster settling, which in turn results in lower switching loss. Lower losses result in higher system efficiency, but also enable the system to operate at higher switching frequencies. Higher switching frequencies help in proportionally reducing the size of the filters in passives, such as inductors, capacitors, transformers, used in a power system.
Higher thermal conductivity results in fewer cooling needs. All these advantages finally resolved in three system level benefits. First, system size reduction. Second, system cost reduction. And third, system weight reduction. Accordingly, all power systems where size, weight, and cost are important performance metrics, silicon carbide turns out to be a perfect material to develop power devices, especially power MOSFETs switches.
Next, I will discuss a few applications that have already adopted silicon carbide MOSFETs based on real life system level benefits as mentioned here. Let us now understand the application scenario for silicon carbide. We have plotted all major power applications in this plot. In the plot, the x-axis denotes the power switch voltage, and y-axis denotes the application power level.
The applications where silicon carbide MOSFETs are getting adopted first in low volume are highlighted in violet, which are, first, rail traction. Second is solar for commercial wind farms. And third is HEV/EV traction inverter and DC-DC modules. Rail traction and HEV/EV benefit immediately from system size and weight reduction. They also benefit from higher temperature operation capability of silicon carbide as compared to silicon IGBTs.
Solar inverters have been [INAUDIBLE] of silicon carbide primarily at very high power levels in the megawatt range, where even a small percentage of efficiency improvement results in huge cost savings with respect to cooling infrastructure. As the cost of silicon carbide MOSFETs reduces over time with economies of scale, we expect silicon carbide to be adopted at higher volume in most of the applications depicted in this plot.
Next, we will investigate the solar and HEV/EV applications in further detail and analyze the system architectures where our silicon carbide is being adopted. Here are typical architectures of utility scale PV system on the top and residential PV system on the bottom. The utility scale PV systems typically support power levels of hundreds of kilowatts up to megawatts, whereas the residential systems support power levels from 5 kilowatts to around 50 kilowatts.
A recent trend is to connect the PV system to a local battery bank where excess power could be stored in the battery. The battery could also provide power to the grid in case the solar panel is not providing enough power. Hence, the battery is connected to the PV system using a bi-directional power transfer mechanism. Both the system architectures have first a DC-DC converter that converts the solar panel power to a DC-link voltage consistent with the next stage input.
The DC-link voltage is typically higher than the voltage provided by the solar panel. Hence, a booster architecture is commonly used for the DC-DC converter. The booster converter is almost always associated with the Maximum Power Point Tracker, also called MPPT, which supplies algorithms to maximize the power to be extracted from the solar panels.
The next stage is typically an inverter that converts the power from the booster converter into single phase or three phase AC power to be transferred to the grid. The inverter helps in synchronizing the output voltage and frequency with the grid, and also provides galvanic isolation. Both the boost converter and inverter stages benefit from using silicon carbide MOSFETs from system size and cost perspective.
Silicon carbide enables better power efficiencies at higher switching frequencies, thus reducing the size of filters and passives, and lowering the cooling infrastructure, thereby reducing the overall system cost. Several utility scale PV systems have been launched as commercial products using silicon carbide MOSFETs. As the cost of silicon carbide MOSFETs reduces with time, we believe silicon carbide MOSFETs would start getting adopted at a higher volume even in the residential PV systems.
Let us now discuss the benefits silicon carbide brings to automotive power systems. Here is a typical power system for Hybrid Electric Vehicles, also called HEV, or pure Electric Vehicles, also called as EV. Vehicles today have a 12-volt bus indicated by the green line at the top left corner, which is typically used to power up circuits, such as entertainment systems and sensors.
Vehicles are migrating to a new architecture that also has a 48-volt bus indicated by the blue line in the middle, which typically powers up several motor units, such as water pumps, cooling vents, power steering, et cetera. Hybrid or pure electric vehicles are also equipped with an on-board battery with the supply voltage as high as 400 volts.
The red light on the right side of the system architecture indicates the high voltage bus, where we focus for silicon carbide applications. The first subsystem that would benefit most from silicon carbide MOSFETs is the traction inverter. The traction inverter converts DC power from the 400-volt battery into AC power to be delivered to the main traction motor that moves the vehicle.
The inverter system also includes a DC-DC converter that regulates the voltage to be supplied to the motor based on its power needs. The traction inverter provides galvanic isolation from the battery to the motor. It has to meet stringent safety standards.
Traction inverters can support a varied range of power levels, depending on the size of the vehicle. For example, from 40 kilowatt up to almost 350 kilowatt in some cases. Because the mileage of the vehicle is inversely proportional to the weight of the vehicle, traction inverters benefit significantly from using silicon carbide MOSFETs, where silicon carbide switches help operate at higher switching frequencies and deliver better power efficiencies, thus reducing the size and weight of the filters, power transformers, and heat sinks required for thermal power dissipation.
Automotive companies have achieved about three to five times size reduction of the traction inverter after transitioning from a silicon IGBT-based inverter to a silicon carbide MOSFET-based inverter. The second subsystem that would benefit from silicon carbide is the on-board charger. The on-board charger converts power from the grid to charge the battery inside the vehicle. It has inbuilt galvanic isolation and includes a power factor correction circuit. The on-board charger mainly benefits from silicon carbide from power density perspective.
The surge subsystems which may benefit from silicon carbide application are the auxiliary power supplies, which may be uni-directional or bi-directional in nature, depending on their purpose. These auxiliary power supplies help transfer power from high voltage battery to low voltage battery, or vice versa. The auxiliary power supplies also benefit from silicon carbide from a power density perspective.
Next, let us discuss a few details regarding traction inverter and on-board charger. Here is an illustration that provides details of the traction inverter needs for the entire range of hybrid electric or pure electric vehicles. The x-axis denotes the amount of electrification, and y-axis denotes the reduction extent of carbon dioxide emissions.
The tables associated with each vehicle type denotes the traction inverter power level in blue, and percentage of electrification in black, and the on-board battery voltage in red. As you may notice, silicon carbide is typically suited for vehicles starting from full hybrid to plug-in hybrid to the pure electric vehicles, because silicon carbide outshines silicon IGBTs, especially at higher battery voltage levels beyond 400 volts.
Let's now understand where silicon carbide finds application in an on-board charger. A typical on-board charger consists of an EMI filter followed by a power factor correction stage, also known as PFC, which is then followed by two DC-DC converter stages that also provide galvanic isolation for safety. The PFC stage converts AC power from the grid to the intermediate DC power at an internal DC voltage.
The DC-DC stage converts the intermediate DC power to the final power used to charge the battery. Both the PFC and the DC-DC power stages significantly benefit from using silicon carbide from power density perspective with increased power transfer efficiencies at higher switching frequencies as compared to those achieved by silicon switches.
On-board chargers are currently limited in terms of their power capability, and hence, take a long time to charge fully. EV faster charging stations reduce recharging time by a factor of 3 to 4 by supplying the DC charging power directly to the battery. Accordingly, the power conversion from grid AC power to DC power occurs outside the car and the charging station.
The power transfer architecture is identical to the on-board charger. However, the power level and the internal power stage topologies vary. The first stage is the AC to DC power conversion. The second stage is the intermediate DC power to battery DC power conversion. This stage also provides galvanic isolation for safety. Both the stages benefit from using silicon carbide from a power density, and hence, system cost perspective, since silicon carbide enables high voltage operation with better power efficiencies and reduce the cooling infrastructure.
Let us now try to understand what it takes to drive the silicon carbide MOSFETs. Below is a list of some of the critical requirements for a silicon carbide MOSFETs driver. Next, we will discuss each of these driver requirements, along with several other new driver requirements specific to silicon carbide in future detail.
Let us discuss some of the new key requirements for silicon carbide for several reasons. First, we would like to most effectively drive the silicon carbide MOSFETs in order to extract the most benefits out of the system in terms of power efficiency. Secondly, we would also need to protect the silicon carbide MOSFETs, and eventually the system, to ensure robust operation over the system lifetime without any malfunctions.
We tabulate the requirement along with the reasoning in a table. The first requirement is to have a high output drive voltage. Silicon carbide MOSFETs, like silicon IGBTs, have thick gate oxide to enable high voltage operation. Accordingly, the MOSFETs have been designed to be typically driven by voltage of around positive 15 volts to positive 20 volts. The extra voltage drive capability ensures that the driver provides a good supply noise immunity and can survive 5 volts to 10 volts of additional supply surges in noisy environments.
Many systems also need to turn off the MOSFET with a negative voltage drive. This is related to what is called the Miller turn-on immunity. Silicon carbide MOSFETs typically have a low turn-on threshold voltage, typically between 3 volts to 5 volts. In case of a switching event, when the gate voltage is pulled down, the rising drain voltage transient could couple into the gate through the gate to drain capacitance of the MOSFETs and transfer enough charge into the gate such that the gate to source voltage rises beyond the turn-on threshold voltage. Such a phenomenon could cause [INAUDIBLE] in a half-bridge topology and eventually cause system breakdown.
To avoid such a scenario, many times silicon carbide MOSFETs are driven with a negative gate to source voltage. With the gate voltage below the source voltage in the turn-off condition, we can avoid the false turn-on event because much more additional charge needs to be transferred into the gate in order to bring the gate potential as high as the turn-on voltage of the MOSFET. Accordingly, a few examples of drive voltages of silicon carbide MOSFETs can be positive 20 volts for turn-on and negative 5 volts for turn-off, or positive 18 volts for turn-on and negative 3 volts for turn-off, et cetera.
High overdrive voltage reduces the ohm resistance of the switch, thus reducing the conduction loss, and hence, increasing the power efficiency. However, the gate oxide typically breaks down sharply beyond a certain voltage, typically around positive 25 volts. Hence, the system needs to ensure the drive voltage never exceeds the maximum voltage.
Silicon carbide MOSFET switch is faster than silicon IGBTs, and hence, provide higher power efficiency. To switch faster, the silicon carbide MOSFETs benefit from higher peak drive currents. This is illustrated by the turn-on switching waveforms shown here.
In the plots, the red waveform depicts the peak drive strengths. The blue waveform depicts the gate to source voltage of the MOSFET. The green waveform depicts the drain to source voltage. And lastly, the orange waveform depicts the drain current of the MOSFET.
In the top event, the switching loss is depicted by the yellow triangle formed by the overlap of the voltage and current waveforms. In the bottom event, a higher peak drive strength results in a faster voltage and current transition resulting in triangle with smaller area, which in turn results in a smaller switching loss. One of the drawbacks of the faster switch transient is the resultant noise generated in the system, which could get coupled to different nodes and could eventually result in system malfunction.
This leads to the next new key requirement of silicon carbide drivers, which is why dV dt immunity, or high CMTI. Silicon carbide MOSFETs are capable of switching at fast rate of greater than 100 volts in 1 nanosecond. Accordingly, the galvanic isolation provided by the driver needs to support even higher CMTI than the rate at which the MOSFETs switch at. TI's industry-best series capacitive isolation technology enables very high CMTI, along with several other advantages, such as the industry's highest lifetimes and low part to part propagation delay variation, which in turn make the system more robust.
Another key requirement of silicon carbide is faster short circuit protection than the silicon IGBT. For the same all resistance, the silicon carbide die is significantly smaller than that of a silicon IGBT, which means that the silicon carbide MOSFETs die has lower thermal dissipation capability. Accordingly, the current surge capability of the silicon carbide MOSFET is lower than that of an IGBT. Hence, silicon carbide drivers have lesser time to detect and turn off the MOSFETs to avoid short circuit breakdown.
Typically, silicon carbide drivers have to detect a short circuit within 2 microseconds to turn off the MOSFET and avoid a catastrophic breakdown condition. The current voltage characteristic of a MOSFET versus IGBT also contributes to this challenge. As illustrated in the below figure, IGBT typical works in the saturation region during the normal on state. When a short circuit happens, the collector current IC increases. Then the IGBT goes through a sharp transition from the saturation region to the active region.
In the active region, the collector current gets self-limited and becomes independent of VCE. Consequently, the increasing IGBT current, and hence, power dissipation gets self-limited. Owning to its behavior, IGBTs have a longer short-circuit handling capability. A desaturation protection circuit is usually adopted to sense the VCE, and the IGBT needs to be shut down when VCE exceeds the pre-set ratio voltage, typically somewhere between 7 volts to 10 volts.
On the other hand, silicon carbide MOSFETs works in a linear region during normal on operation, acting more like a resistor. During a short-circuit event, the silicon carbide MOSFET enters the saturation region. Silicon carbide MOSFETs have a larger linear region different than that of an IGBT.
The transition from linear region to saturational region happens at significantly higher drain to source voltage. Because of the higher transition voltage, many times the transition from linear to saturation region never occurs, which means that the drain current cannot self-limit itself, and the current keeps increasing along with increasing drain to source voltage. Accordingly, silicon carbide MOSFETs have significantly reduced the short-circuit capability as compared to two IGBTs, and hence, need to be protected quickly, typically within 2 microseconds.
In many cases, the traditional desaturation circuit used for IGBT is no longer suitable for silicon carbide MOSFET because the desaturation circuit is associated with a [? long ?] planking time. At TI, we are working on fast and accurate overcurrent detection techniques for silicon carbide MOSFETs. Our drivers would protect the necessary flawless overcurrent detection, protect the MOSFETs quickly, providing the system designers their peace of mind.
To improve the system efficiency, smaller propagation delay and deliberation are required for silicon carbide MOSFET driver. During the dead time, the current can be conducted through the body diode of the MOSFET. Silicon carbide MOSFET body diode has a relatively large voltage drop, which causes significant energy loss in each switching cycle. The minimum dead time depends on the pulse width distortion of the input signal, where the pulse width distortion is determined by the propagation delay mismatch of the rising edge and the falling edge.
Smaller propagation delay leads to smaller dead time, hence reduced power loss through the silicon carbide MOSFET body diode. Additionally, the system control response time also gets reduced due to the smaller propagation delay of the driver. Silicon carbide driver can have a switch temperature sensing feature to provide thermal shutdown for the MOSFET.
The threshold temperature can be preset to shut down the device at a certain temperature in applications requiring advanced switch protection. It is also critical to prevent a silicon carbide MOSFET from overheating in high temperature applications. For example, in electric vehicles and oil drilling machines.
Let us now discuss some of the TI's industry-leading silicon carbide driver products. The UCC2753X family is a single channel, non-isolated driver product family with 35 volts output drive voltage, 2.5 amps peak source current, 5 amps peak sink current, very low propagation delay of 17 nanoseconds.
It has the industry's best negative handling capability on its pins, which makes it ideal for operating flawlessly in noisy environments. It comes in tiny, 6-pin SOT-23 package that saves for space in end equipments. These products are also available in automotive qualified versions.
The UCC2152X family is a leading dual-channel isolated driver product family with 25 volts output drive voltage, high peak drive strengths of 4 amps source and 6 amps sink currents, industry's lowest propagation delay of 90 nanoseconds. The product can support either reinforced isolation standard, i.e., 5.7 kilovolts RMS for 60 seconds, or basic isolation standard, i.e., 3 kilovolts RMS for 60 seconds, meeting the IEC and VDE standards.
TI's industry-best capacitive isolation technology also enables very high CMTI capability beyond 100 volts per nanosecond. The accurate capacitive isolation barrier also enables very tight propagation delay matching between the two driver passes, which enables paralleling the driver to achieve twice the driver strengths in case of driving large silicon carbide MOSFETs or power modules. The dual channels are isolated to each other, and hence, could also be used for high voltage half-bridge applications. The driver also features a unique externally programmable overlap and dead time control that enables higher power efficiencies for faster switching power converters.
The UCC53XX family is a single channel isolated silicon carbide driver product family with up to 33 volts of output drive voltage, with 2.2 amps, 4.4 amps, and 17 amps peak drive strength. The 17 amps peak drive strength is perfectly suited for driving silicon carbide power modules. The high drive strength significantly reduces the transient switching losses by speeding up the voltage for current transitions. The UCC53XX products also feature a Miller clamp required for a robust operation.
The ISO58XX family is a single channel isolated silicon carbide driver with advance protection features, such as active Miller clamp, desaturation detection circuit for fast overcurrent detection, softer turn-off for smooth system shutdown in case of short circuit event, and isolated overcurrent fault reporting to the controller. The driver features a high peak drive strength of 2.5 amp source and 5 amp sink. These products are also available in automotive qualified versions.
Apart from the available driver products, we're developing leading edge silicon carbide specific drivers to improve upon all the new key requirements I mentioned earlier. Here are several silicon carbide driver evaluation modules available at ti.com, which you could order to evaluate our latest isolated silicon carbide driver products. The links will take you directly to each of the evaluation model details web page.
Thank you for your attention. I encourage you to review the below references to learn more about how TI's leading edge technologies are enabling industry-best silicon carbide driver products. Thank you.

Description

October 9, 2017

SiC MOSFET is penetrating the electrical applications thanks to its superior material properties. The features of SiC material can realize system size, cost and weight reduction. SiC MOSFET can be utilized in power systems such as hybrid electric vehicles/electric vehicles (HEV/EV), solar inverter systems, motor drives, power converters, etc., operating in the range of hundreds of Watts to MegaWatts. Driving SiC MOSFET in the right way can fully unleash the intrinsic material advantages. By understanding the requirements of driving a SiC MOSFET and the system benefits realized, system manufacturers could achieve much more efficient, compact and reliable systems.